NRCS Conservation Drainage Practices Part 1: Saturated Buffer

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Natural Resources Conservation Service Helping People Help the LandHelping People Help the Land

Presented by: Ruth Book, P.E., Ph.D.

State Conservation Engineer

Champaign, Illinois

www.il.nrcs.usda.gov

Illinois Sustainable Ag Partnership – June 14, 2018

NRCS Conservation Drainage Practices

Part 1: Saturated Buffer

Agenda

What the saturated buffer looks like and how it works Meeting the standard

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Natural Resources Conservation Service Helping People Help the Land

Saturated Buffer

“Leach” tile water into the soil Let the soil provide the media for

denitrification

Let the vegetation uptake some

nutrients

Saturated Buffer Contents

A water control structure (near the end of a tile line) Distribution line(s)

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Schematic

Conventional Outlet Saturated Buffer

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Agenda

Determining feasibility

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Conditions Where Practice Applies

Land with subsurface drainage (that can be adapted to discharge into a

buffer)

Where a raised water table can be maintained without adversely affecting

banks, neighbors

Does NOT apply to any system with surface inlets! Not to be used for septic effluent or animal waste

Criteria Development

Results Number of Sites

Substantial nitrate removal 4

Promising in at least one year (2013-2015) 3

Insufficient data 3

Failure – did not remove nitrate 5

Several reasons for the failures:

• Coarse soil layers (couldn’t maintain a water table)

• Inadequate soil carbon (no energy source for denitrifying bacteria) • Improper design or installation • High water levels in ditch

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General Criteria

Minimum width of vegetated buffer zone 30 feet

The vegetated buffer zone is defined as the area between the distribution pipe and the receiving channel on which permanent vegetation is maintained.

General Criteria

Maximize “bang for the buck”

(locate so you can treat as much subsurface drainage water as possible)

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Unsuitable Site vs Suitable Site

At least 1.2% O.M. in top 2.5 feet Not much organic matter WATER WATER WATER Clay or moderately impermeable layer Coarse gravel layer

Preplanning - Soil

Presence of Organic Matter Capable of holding a water table

Poorly or somewhat poorly drained Absence of sandy or gravelly

layers?

Hydraulic properties

Ksat

Drainable porosity

http://websoilsurvey.sc.egov.usda.gov

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Criteria – Geologic/Soil Investigation

Possible to hold a water table

At least 1.2% organic matter in top 2.5 feet Abandoned pipes or tile?

Depth to restrictive layer Hydraulic conductivity

…and visual observation

Saturated Hydraulic Conductivity

Web Soil Survey? Guelph Permeameter Other methods

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Criteria - Bank and Channel Stability

Visual assessment:

Bank issues

Downcutting channel Lateral migration potential? Incised >8 ft?

Q&A: What if the banks are > 8ft deep?

Clay soils with Plasticity Index (PI) = 30 to 40

are stable on ~ 3:1 slopes

Clays with PI>80 need more like 6.5:1

NEH 654, TS 14A

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So, what now?

Geotechnical engineering is beyond the scope of this session.

If you have a site with banks > 8ft and our triage indicates you might have

a problem, consider one of these options:

Find a different site or pick a different conservation drainage option

Lay the bank back at a more stable slope (set the distribution line far enough back)

Stabilize the site first (CPS 580 – Streambank and Shoreline Protection) Involve a geotechnical engineer

Use visual assessment option

Slope Stability Visual Assessment

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Criteria - Flow

Minimum design flow 5% of max capacity of drainage system Or as much as is practical based on available length of vegetated buffer

How to determine if the buffer has adequate capacity Use soil saturated hydraulic conductivity and hydraulic gradient Other methods

Some history on the Capacity criterion

Started as 15% (like the bioreactor)

Very restrictive…very few sites would be able to meet the criterion

Capacity is determined by how much flow you can leach out through the soil from the distribution pipe (longer = more flow)

Why have capacity criteria? ECONOMICS

Saturated Buffers

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Special Note on Capacity

Minor update to national standard (October, 2017)

Determining Drainage System Capacity

1. Mainline configuration (tile size, type, grade  capacity of outlet main)

2. Drainage Coefficient (Q = DC inches/dayx acres drained)

 With tile map (known drained area)  Without tile map (estimated drained area)

3. Modeling

 Library of DRAINMOD runs for typical soil textures  Site specific modeling

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Drainage System Capacity – Option 1

Mainline configuration

Tile size and diameter Tile grade

Roughness coefficient

1.486

Tends to be conservative

Drainage System Capacity – Option 2

Drainage Coefficient method

With tile map – determine acres drained by the system

Find the drain spacing for the soil type and estimated depth, from the state

Drainage Guide, and divide in half (½S)

Delineate drained acres by drawing a line around the tile system, ½Son each side

of the tile.

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Option 2 - Example

Given:

Random tile system 3,075 ft long including lateral. Harpster soil: tile

spacing for 48” depth 80-90 ft. Typical Illinois drainage coefficient = 3/8” per day.

Find drainage system capacity:

first, find area drained Area drained, ft2_{= length x spacing}

Area drained =

Option 2 - Example

Given:

Random tile system 3,075 ft long including lateral. Harpster soil: tile

spacing for 48” depth 80-90 ft. Typical Illinois drainage coefficient = 3/8” per day. Remember our drained area calculation just now.

Calculations:

now find capacity of tile

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What if there is no tile map?

Tile outlet is here

Acres Drained by Tile – Option 2, Method 2

Tile outlet is here

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Acres Drained by Tile – Option 2, Method 2

If you can’t get a map of the tile system,

delineate the entire watershed for surface flow to the tile outlet.

Acres Drained = all of the watershed

acres with poorly or very poorly drained soil, and half of the acres with

somewhat poorly drained.

Use Web Soil Survey to determine the

soils in the watershed, and how many acres of each.

http://websoilsurvey.sc.egov.usda.gov

Option 2 – Example with no tile map

Given:

Typical Illinois drainage coefficient = 3/8” per day Soils in watershed:

Find drained area and drainage system capacity:

Map Unit Soil Name Drainage Class Acres

51A Muscatine Somewhat poorly drained 13.7

86B Osco Well drained 38.2

3107A Sawmill Poorly drained 51.9

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Design Flow

5% of Drainage System Capacity…figured whichever way you determine is appropriate

EXAMPLE:

Given:Drainage system capacity = 0.93 cfs Calculation:

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How to achieve the design flow?

Unit flow per foot of “leach” pipe

Impervious layer closer to surface

means less unit flow

Maintain water table within 12” of

ground surface @ distribution pipe Board setting = h₁ Base flow = h2 2 impervious layer

Criteria - Water Control Structure

Water control structure (CPS 587) similar to drainage water management Nonperforated pipe for overflow

Avoid draining saturated soil zone Comply with velocity criteria in CPS 606

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Criteria - Distribution Pipe

Meet criteria for CPS 606 – Subsurface Drain Target is flow uniformity

We analyze this 3-D problem in 2-D (at the main water control structure)

Add structures as needed (3’ max elevation difference between structures)

Other Distribution Pipe Layout Options

Level

Drain to structure

2 ft min cover

Lower Δ head from pipe to stream

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Preplanning - Topography

Q & A: What if we can’t get the uniformity?

Divide the analysis into reaches

Similar distance from stream (± 10%~?)

Similar change in elevation from distribution pipe to stream

(± 1.5 ft)

Consider a different

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Sizing the Distribution Pipe Diameter

The limitation needs to be flow through the soil, not flow through the pipe.

Think of it as a manifold with much larger potential input than output.

IL standard drawings show 6” distribution pipe. This should be adequate

for most situations.

Sizing the Distribution Pipe Length

Assumes reasonably constant head differential and soil properties along

the entire reach

Need: at least 5% of system capacity

Limitation will be flow through soil. How long will the pipe need to be?

Given a unit flow, we can get more flow with a longer distribution pipe.

2

q = flow per unit length of distribution pipe

L = horizontal distance from pipe to receiving channel h1= head at flash board setting (height above impervious layer)

h2= head at base flow in channel

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Example – Distribution Pipe Length

Given:

Required design flow to achieve 5% of drainage system capacity is 0.026 cfs(33 acres, 3/8” drainage coefficient).

Calculations:

Unit flow through soil q = 0.099 ft3_{/hr per ft} _{(from previous calculations)}

Total flow required (ft3_{/hr) =}

Distribution pipe length, ft = =

Criteria - Vegetative

Can use Critical Area Planting (342) or Conservation Cover (327) If you’re in an existing filter strip, check to see what the revegetation

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Set it and forget it?

If topography allows, set saturated buffer lower in

elevation so it can operate year round without affecting field trafficability.

Q & A: Can you do DWM with the saturated buffer?

Yes!

Suites of practices are highly recommended.

If the site is relatively flat, some measure of DWM will likely be required. May be able to hold back water and nutrients for crop production.

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Plans and Specifications

Plan view

Profiles: existing drain, distribution pipe, outlet channel Structural details

Vegetation establishment requirements Construction specs

See the

Statement of Work

for more details.

Operation and Maintenance

Management information (water levels and timing)

Inspection and maintenance requirements (both SB and contributing

drainage system)

Periodic removal of invasive trees/shrubs to reduce plugging Performance monitoring (if planned)

Demonstrate system performance Improvements needed

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Agenda

Determining feasibility

Inventory & Evaluation (“I&E”)

Is the site appropriate for a saturated buffer?

If not, is there a better site nearby?

Or is there a more suitable conservation drainage practice?

Plan maps and quantity estimates so client can make planning decisions

Client preferences

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Saturated Buffer needs to fit the site

Each site is different Factors to consider:

Control elevation Bank stability

Property lines and infrastructure Available buffer width (existing or

proposed)

What is this?

Q&A: Any sites we should avoid?

Subsurface drainage system must

have NO surface inlets

Soils need to be able to hold a water

table and have enough organic matter

Bank/channel must be stable

Avoid locations where restrictive layer

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Q&A: Any sites we should avoid?

Minimize flooding from

receiving channel

Avoid flooding the crops! (and neighbors)

Avoid locations within drip

line of trees (to reduce root plugging)

Saturated Buffer I&E info needed – basic

ID info (county and client name, legal description/location) Is the land currently under a conservation program? Preferences of the client

Active or passive management? Open to doing DWM as well?

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Saturated Buffer I&E info needed - technical

Maps

Aerial, with property boundaries, location of proposed practice Topography/soils with drainage area delineated (LiDAR preferred) Tile (or at least tile dia, depth, grade, location, surface intakes)

Site conditions

Vegetation on site (photos?) Crops/ proximity

Elevations (baseflow, crop, proposed buffer site, etc) Receiving channel/ area (bank stability)

Geologic investigation

Any abandoned tile we need to be aware of?

NRCS Resources

Related standards

Structure for Water Control (Code 587) Subsurface Drain (Code 606)

Drainage Water Management (Code 554)

Guidance document “how-to” Standard drawings

Customizable construction specs and

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